Evaporative coolers, both direct and indirect, are well known in the art. Evaporative cooling is generally considered energy efficient because fans, rather than more energy intensive pumps used in traditional vapor compression systems, power the cooling processes. In dry desert and high altitude climates, direct evaporative cooling alone provides energy efficient cooling in popular commercially available products, commonly referred to as "swamp coolers." In humid regions, however, these direct evaporative coolers are less effective as they will not appreciably lower the conditioned air temperature and they increase room humidity when the room air is already laden with moisture.
Direct evaporative coolers remove heat while increasing the moisture in the air. In general terms, direct evaporative cooling dissipates the heat in air, using the heat energy in the air to evaporate water. The energy consumed to evaporate the water in evaporative coolers reduces the air temperature. In direct evaporative coolers, a fan is used to blow warm outside air through a water absorbent medium such as an open cell foam, corrugated cardboard or wood filings. When the warm air passes the water absorbent medium, the water evaporates, causing the air to become cooler, but more humid. This process is called adiabatic cooling because no energy is lost. This loss of sensible heat energy (generally expressed as temperature) causes the vapor pressure in the air (generally expressed as humidity) to rise. The energy that causes the humidity to rise is called latent heat energy and the reduction in sensible heat equals the increase in latent heat.
When hot, dry air enters the direct evaporative cooler, it can only be cooled to the wet-bulb temperature. As air is cooled, the relative humidity increases. When air is cooled to the point where it will hold no more water (i.e., 100 percent relative humidity) that temperature is referred to as the dew point. The wet-bulb temperature is the lowest temperature that can be measured on a water absorbent surface when air is flowing past at high velocity. The dew point temperature is often well below that of the wet-bulb temperature, particularly when the air is hot and dry. The dew point is the temperature at which liquid within the air will condense.
Indirect evaporative cooling extends the usefulness of evaporative cooling to more humid climates. In an indirect evaporative cooler, the evaporative cooling process is separated from the occupants of the building or enclosed structure by a heat exchanger that conducts heat. On one side (hereinafter, the "wet" side) of an indirect evaporative cooler, contained within the heat exchanger, a first stream of air blows through a water filled medium or air spray, lowering air temperature and increasing humidity in a way similar to the direct evaporative cooler. On the other side of the cooler (hereinafter, the "dry" side), a second stream of warm air blows past the heat exchanger, cooling the second stream without adding moisture to the air. In this way, humid air can pass the wet side of the heat exchanger without changing the moisture content of the air cooled on the dry side of the heat exchanger. In other words, indirect evaporative coolers work by dividing a first air flow into a first and second portion, then by directing the first portion through one or more wet internal pre-cooling stages, lowering the temperature within the last stage of the cooler to a level that approaches the dew point of the air at entry. Heat in the air on the dry side of the cooler is transferred to the wet side of the cooler, such that the temperature of the air leaving the dry side of the cooler also approaches the dew point temperature of the air at entry.
Indirect evaporative cooling is more effective than direct evaporative cooling in more humid climates because indirect methods do not add moisture to the air. However, as described above, relative humidity increases when air is cooled so there is a limit to the effectiveness of indirect evaporative cooling. This is especially true in more humid climates.
Evaporative coolers typically employ moisture absorbing surfaces that must be cleaned or replaced at regular intervals. Indirect evaporative cooling systems are deficient in providing access to such moisture absorbing surfaces for service or replacement. Many systems have a limited life span due to the deteriorating effects of water on the moisture absorbing surfaces and the lack of service access to such surfaces. Accordingly, a need exists for indirect evaporative coolers to be easily serviceable.
Desiccant air drying techniques, used in concert with evaporative cooling or existing vapor compression systems, are used to provide deep dehumidification in certain applications where this is desirable. Supermarkets, for example, have employed desiccant air drying as it is seen as beneficial for reducing spoilage in perishable food storage. However in practice, these systems have yet to achieve efficiencies substantially greater than existing vapor compression systems. This is largely due to the mechanical complexity of the complete system and the continuous energy demand of the desiccant and enthalpy wheels common in such systems.
Conventional desiccant/evaporative cooling systems also have a disadvantage in that they are relatively large and expensive, due to their mechanical complexity. For example, a typical desiccant wheel alone may be four feet in diameter and one foot in depth. This drawback limits the acceptance of conventional systems in markets where initial cost and package size is a major factor, such as residential and transportation markets.
Theoretical efficiencies for air conditioners which use desiccant dehumidification in combination with evaporative cooling have been estimated at up to five times that of conventional vapor-compression air conditioning systems. Additionally, desiccant/evaporative cooling systems offer an advantage over vapor-compression systems in that they continuously pass a large volume of air through an enclosed structure resulting in more fresh air being introduced into an enclosed structure. Accordingly, the air quality is improved along with efficiency.
A basic psychrometric cycle for desiccant/evaporative cooling systems was devised by Pennington and the cycle is known as the Pennington or ventilation cycle (see FIG. 2). With reference to FIGS. 1 and 2, the cycle involves drying an incoming ambient airstream 100 with a desiccant wheel 140 to produce a second airstream 104 which is dryer and hotter. The second airstream 104 is then cooled with an air to air heat exchanger 144 to produce a third airstream 108. Typically, the air-to-air heat exchanger 144 is implemented with a rotating wheel assembly, known in the art as an enthalpy wheel. After cooling in the heat exchanger 144, the third airstream 108 is passed through a first direct evaporative cooler 148 to cool and humidify the third airstream to produce a fourth airstream 112 which is introduced into an enclosed structure 152. In this way, cool air is introduced into the enclosed structure 152.
The fourth airstream 112 is heated by such sources as solar radiation striking the exterior of the structure and/or the presence of people and humidified by such sources as cooking, industrial activities and/or people within the enclosed structure 152 to generate a fifth airstream 116. The fifth airstream 116 passes through a second direct evaporative cooler 156 where it is cooled and humidified to produce a sixth airstream 120. The sixth airstream is passed though the air-to-air heat exchanger 144 which cools the second airstream 104. In the process of cooling the second airstream 104, the sixth airstream 120 is heated to produce a seventh airstream 124. In a heater 160, the seventh airstream 124 is heated to produce an eighth airstream 128. The eighth airstream 128 is used to regenerate one side of the desiccant wheel 140 before being exhausted as a ninth airstream 132.
Conventional Pennington cycle desiccant/evaporative cooling systems typically utilize a continuously-regenerated desiccant wheel 140 to remove latent heat from a confined area, together with one or more evaporative coolers 148 to remove heat and humidify. In other words, the incoming airstream is first dehumidified and then cooled which extends the area in which an evaporative cooler is practical to include more humid climates. The desiccant wheel 140 is generally divided into a first portion 168 which removes humidity from incoming ambient air and a second portion 164 which is regenerated by heating the desiccant with a warm airstream 128. While the desiccant wheel 140 rotates, the first portion 168 is constantly changing so that it is regenerated in the second portion 164. As those skilled in art can appreciate however, rotating the desiccant wheel 140 to constantly regenerate the desiccant consumes energy. Further, use of the enthalpy wheel as the air-to-air heat exchanger 144 consumes additional energy. Consumption of energy in this way, reduces the efficiency of desiccant/evaporative cooling systems.
In summary, it would be desirable to develop a desiccant/evaporative cooling system which: (1) improves upon the Pennington cycle to more efficiently cool an enclosed structure, (2) does not use desiccant or enthalpy wheels because of their excessive energy demands, (3) allows for easy serviceability of the evaporative coolers, and (4) is not mechanically complex such that applications which require less space may use the cooling system.